A terahertz region of the spectrum of electromagnetic radiation (T-Rays or THz) is located between the most long wavelength “optical” part of the spectrum, i.e. the infra-red light, and the most short wavelength part of the “radio” spectrum, known as microwaves. The terahertz spectral region thus encompasses the frequencies in the range of about 0.1 through 20 THz or the wavelengths in the range of about 15 μm through 3 mm, although it should be appreciated that these limits are indicative rather than absolute.
The terahertz spectral range has an extreme importance owing to the variety of applications where it can be utilized. One important application of the terahertz spectral range is related to various imaging techniques (medical imaging, technological imaging, or security imaging), where there is a trend for a switch from the harmful and, sometimes, lethal X-rays to T-rays (see, for example, U.S. Pat. Nos. 5,710,430 to Nuss and U.S. Pat. No. 5,894,125 to Brener et al.)
Terahertz radiation can penetrate non-polar substances such as fats, cardboard, cloth and plastics with little attenuation. On the other hand, materials including organic substances have varying responses (transmission, reflection and absorption characteristics) to terahertz radiation. Likewise, water molecules absorb terahertz waves, on the one hand limiting penetration of the radiation in moist substances, and on the other hand making it readily detectable even in very low concentrations. Accordingly, use of terahertz radiation can indicate the presence of different materials in a medium.
T-rays are strongly attenuated by moist tissue, because of water absorption. However, having low average power, i.e. relatively low ionizing capability, T-rays are particularly attractive for medical applications where it is important to avoid damaging a biological sample.
Another important application of terahertz radiation is related to the communication technology. This can be the terahertz range that is the nearest and the most important barrier in the way to increase the bandwidth of wavelength-division-multiplexed communication networks.
Recent achievements in both fields mentioned above are rather remarkable, but still limited. The key reason for this limitation is a lack of reliable THz sources and detectors, especially when compared with the neighboring frequency ranges of microwaves and infrared radiation.
U.S. Pat. No. 6,476,411 to Ohno et al. describes a luminescent element that consists of indium-arsenide (InAs) and gallium-antimony (GaSb) semiconductor layers formed with specified band gap. The first layer makes a heterojunction with the second layer. The top of the valence band of the first semiconductor material is higher in energy than the bottom of the conduction band of the second semiconductor material. The element further includes a third layer making a heterojunction with the first or second layer. The third layer has a superlattice structure. One of the first and second layers is provided on the semiconductor substrate directly or through at least one semiconductor layer.
U.S. Pat. Application No. 2003/0127673 to Williamson et al. describes a semiconductor epitaxial structure optimized for photoconductive free space terahertz generation and detection. The epitaxial structure, termed as a photoconductive gate, includes a substrate composed of GaAs. A barrier layer is disposed between the substrate and photoconductive layer. A bipolar terahertz antenna comprised of a first pole and a second pole is disposed on the photoconductive layer. Sampling of a free space terahertz waveform occurs when the illuminated photoconductive gate conducts for a time shorter than the entire terahertz wave cycle. During the conduction period, charge flows from one side to another of a dipole antenna structure due to the potential difference induced by the terahertz wave. The amount of current flow per sampling optical pulse is proportional to the terahertz voltage potential and the off-state resistance of the interaction area.